Granite {Quartz-felsite } Rhyolite and } { ('quartz-porphyry')} Obsidian } Felstone } } } Syenite {Orthoclase-porphyry } Trachyte } } Claystones } Diorite {Hornblende-porphyry } Andesite Porphyrite } Miascite {Liebnerite porphyry } Phonolite ? -- Gabbro {Augite-porphyry and } Basalt Melaphyre Wacke {Dolerite } Some petrographers, indeed, have maintained the principle that rocks belonging to widely separated geological periods, even when they exhibit no essential points of difference, should nevertheless be called by distinct names. But such a system of classification is calculated rather to hinder than to advance the cause of science. If the palæontologist were to adopt the same principle and give distinct names to the same fossil, when it was found to occur in two different geological formations, we can easily understand what confusion would be occasioned, and how the comparison of the fauna and flora of the different formations would be thereby rendered impossible. But the naturalist, in his diagnosis of a species, wisely confines himself to the structure and affinities of the organism before him; and in the same way the petrographer, in giving a name to a rock, ought to be guided only by his studies of its chemical composition, its mineralogical constitution, and its structure, putting altogether out of view its geographical distribution and geological age. Only by strict attention to this principle can we hope to arrive at such comparisons of the rocks of different areas and different periods, as may serve as the basis for safe inductions. Before leaving this question of the relation which exists between the igneous rocks of different ages, it may be well to notice several facts that have been relied upon, as proving that the several geological periods are distinguished by characteristic igneous products.